(1) Field of the invention:
The present invention relates to a digital tape measure in which a tape is stored in a case and is movable being drawn out of the aperture of the case, and the detected volume of movement of the tape is converted to an electric signal so that the measured length may be digitally displayed being converted to an optional system of units.
(2) Prior art:
In a known digital tape measure wherein the detected volume of movement of the tape is converted to the electric signal by using a photoelectric transfer element, a light transmitting slit is provided on the tape, the tape is inserted between light emitting elements and light receiving elements, and the volume of movement is converted to the electric signal by reading the brightness, thereby the measurement of length being performed as is disclosed in Japanese laid open Patent Publication (unexamined) No. 49-33754.
In another known digital tape measure, a light is projected on a mark designated on the tape in place of providing a light transmitting slit thereon, and the detected signal corresponding to the volume of movement of the tape is counted by receiving the reflected light, thereby the measurement of length being performed. Furthermore, there has been proposed such digital tape measure as detecting a variation in the quantity of light by moire fringe for the purpose of securing high measuring accuracy as is disclosed in Japanese laid open Patent Publication No. 53-83653. Among the foregoing known digital tape measures, the first one in which a light transmitting slit is provided on the tape has a disadvantage that the straight extensibility and the strength against bending (bending resistance) both indispensable to the tape itself are unavoidably lowered, which is not preferable in view of the practical use. In case of said second one in which the measuring is performed using the reflected light from the tape on which a light is projected, although the accuracy in reading the mark is improved by stopping the focus of emitting light and receiving light, a lens system is necessary for stopping the focus resulting in the increase in cost. Moreover, since such focus stopping is performed, a miscount is possibly caused by small dust or stain on the tape negatively affecting the reliability of the measured result.
In case of the third one in which a variation in the quantity of light is detected using moire fringe, a mask is necessary, and considering that with respect to some object to be measured by means of an ordinary digital tape measure, such method of reading as using moire fringe is not always necessary, an increase in cost thereof is unavoidable in the same manner as the aforementioned second tape measure.